This invention relates to neutron sources and more particularly to accelerator-type neutron tube sources having improved ionization sections.
Accelerator-type neutron tube sources are employed in many applications. A well known application is in the radioactivity logging of wells penetrating subterranean formations. For example, in the art of neutron-neutron well logging a source of primary neutrons is employed to irradiate subterranean formations of interest. The resulting secondary radiation is measured by one or more detectors spaced axially from the source within the borehole. Such secondary irradiation may take the form of thermal neutrons, epithermal neutrons, or thermal neutron capture gamma rays. A logging tool of this type employed for porosity measurements is disclosed in U.S. Pat. No. 4,005,290 to Allen wherein the logging tool includes a neutron source and epithermal and thermal neutron detectors.
In procedures such as porosity logging, the neutron source is a continuous source usually of a chemical type. Other well known radioactive well logging techniques involve the use of pulsed neutron sources. For example, in the art of radioactive assay well logging an assay tool is lowered into the well to the level of a formation to be assayed. The assay operation is then carried out by cyclically operating a neutron source in the tool in order to irradiate the formation under investigation with repetitive bursts of fast neutrons. In one assay procedure, disclosed in U.S. Pat. No. 3,686,503 to Givens et al, delayed fission neutrons emitted by uranium within the formation may be detected by a neutron detector. Another procedure, disclosed in U.S. Pat. No. 4,180,730 to Givens et al, involves detection of prompt fission neutrons emitted from uranium in the formation. Pulsed neutron logging techniques may also be employed in procedures in which radioactive decay rates are determined. Thus, the formation under investigation is irradiated with a burst of fast neutrons and the resulting neutron population is detected during successive or overlapping time windows. For example, U.S. Pat. No. 3,800,150 to Givens discloses a pulsed neutron logging technique in which epithermal neutron decay or thermal neutron decay is measured by employing time windows for detection which partially overlap one another.
Neutron sources such as may be employed in radioactive logging procedures as described above may take the form of accelerator-type neutron tubes comprising a target section, a replenisher section, and an ionization section located between the target and the replenisher section. The replenisher section provides a source of accelerator gas to the ionization section where it is ionized and then accelerated to impact the target. The target is formulated of material which responds to the bombarding ions to produce neutrons. In a number of well known accelerator-type tube sources, heavy isotopes of hydrogen are employed as the accelerator gas and in the target. For example, the accelerator gas may take the form of deuterium or mixtures of deuterium and tritium and the target may include tritium molecules, deuterium molecules or mixtures of deuterium and tritium molecules. The so-called deuterium-tritium nuclear reaction is one commonly employed in an accelerator-type neutron tube to produce neutrons. In the replenisher section a filament or reservoir usually made of zirconium or titanium is electrically heated (under controlled conditions) to release deuterium gas previously adsorbed in the filament or reservoir. Zirconium and titanium have the property of adsorbing copious quantities of different gases such as hydrogen, deuterium, tritium, and other gases. These materials have the further property of releasing the hydrogen isotope gases under a controlled release condition when heated to about 300.degree. C. and at the same time retaining other gases that may have been adsorbed. The deuterium molecules are ionized in the ionizing section by the application of a positive voltage to an anode in the ionizing section. The deuterium ions are then accelerated by a large negative voltage, e.g. -100 KV, and impact the tritium target to produce a supply of neutrons.
While various techniques may be employed in ionizing the accelerator gas, one ionization technique which is suitable particularly where the neutron source is operated at a low accelerator gas pressure and in a pulsed mode is the so-called Penning method. A Penning ion source comprises spaced cathodes and an anode located intermediate the cathodes. In a cold-cathode type Penning ion source, electrons are emitted from a cathode surface by field emission when a positive voltage pulse is applied to the anode. A magnet associated with the source functions to spiral the electrons thus increasing their flight path and increasing the statistical probability that they will collide with molecules of accelerator gas supplied to the ionization chamber. In a well designed Penning ion source, some of the electrons originating at one cathode surface will impact the other cathode surface and secondary electrons are emitted which also function to increase the ionization reaction. Such ion sources are well known to those skilled in the art and are described in Flinta, J. "Pulsed High-Intensity Ion Source", Part I; Pauli, R. and Flinta, J. "Pulsed High-Intensity Ion Source", Part II, Nuclear Instruments 2, pp 219-236 (1958). In a hot-cathode type Penning ion source, one cathode is a heated filament and initial electrons are supplied by thermionic emission from the filament. In all other respects, cold-cathode and hot-cathode Penning ion sources are essentially the same. Hot-cathode ion sources are also well known to those skilled in the art and one such source is described in Wood, J. and Crocker, A. "An Electrostatically Focused Ion Source And Its Use In A Sealed-Off D.C. Neutron Source", Nuclear Instruments And Methods 21, pp 47-48 ( 1963).